When a person is being fitted for a new hearing aid, such as the hearing aid disclosed in U.S. Pat. No. 4,425,481 (Mangold et al., 1984) it is often necessary to measure sound levels (commonly called the "sound pressure level," or SPL) in that person's ear canal in order to properly select parameter values for use in the hearing aid. Furthermore, in order to perform such sound level measurements, it is generally necessary to insert a probe tube into the person's ear canal (i.e., the external auditory meatus) for conducting sound from the measuring point to a microphone. The microphone detects the sound levels and generates an electrical signal which is transmitted to a system for measuring the detected sound levels. The receiving system uses the measured sound levels to calculate parameter values for use in a hearing aid.
While measuring the sound pressure level in a person's ear canal, it is important to control the position of the probe tube which conducts sound from the measuring point in the ear canal to a microphone. Variations in this position directly affect the measured sound level, which will affect the correctness of the measurements and the hearing aid gain parameters calculated using those measurements.
Another aspect of measuring sound pressure levels in the ear canal is the use of a reference microphone. Reference microphones are used in a variety of standard measurement methods, including the methods known as the "comparison method", the "pressure method" and the "modified pressure method". The comparison method requires that the test microphone and the reference microphone, employed to measure the free field sound pressure, be placed simultaneously at two acoustically equivalent points in the sound field, i.e., in each of the two ear canals. The pressure method uses a pressure-calibrated reference microphone at a point close to the entry of the ear canal to control the input sound pressure level produced by a sound source, e.g., a loudspeaker, to eliminate diffraction effects. Modified pressure methods differ from the pressure method only in that the reference microphone is placed near the earlobe rather than at the precise location of the opening of the ear canal. These methods are described by Poul B. Madsen, "Insertion gain optimization," Hearing Instruments, vol. 37, no. 1, pp 28-32 (1986); David A. Preves and Roy F. Sullivan, "Sound field equalization for real ear measurements with probe microphones," Hearing Instruments, vol. 38, no. 1, pp 28-32 (1987); and Harvey Dillon and Narelle Murray, "Accuracy of Twelve Methods for Estimating the Real Ear Gain of Hearing Aids," Ear and Hearing, vol. 8, no. 1, Williams & Wilkins Co. (1987). The aforementioned references are hereby incorporated by reference in their entirety.
The preferred embodiment of the present invention includes a reference microphone in a position appropriate for use in a modified pressure method of measuring sound pressure levels in the ear canal.
The prior art includes a number of systems for holding a probe tube in a fixed position in a person's ear canal. These systems generally use hooks over the pinna (i.e., the external ear), and/or headbands encircling all or part of the head. Furthermore, these systems are generally cumbersome, and are too limited in flexibility to be easily used with all patients, or require elaborate mechanical arrangements (e.g., gimbals) in order to change the position of the probe tube.
It is therefore an object of the present invention to provide an improved ear probe holding apparatus which is mechanically simple and provides improved capabilities in terms of positioning and maintaining the position of a probe tube in a person's ear canal. Another object of the present invention is to provide an ear probe holding apparatus that also holds a reference microphone in close proximity to the opening of the ear canal so that the reference microphone can be used to control the sound pressure level close to the opening of the ear canal.